This invention relates to the administration of continuous positive airway pressure (CPAP) treatment for partial or complete upper airway obstruction.
In the Sleep Apnea syndrome a person stops breathing during sleep. Cessation of airflow for more than 10 seconds is called an xe2x80x9capneaxe2x80x9d. Apneas lead to decreased blood oxygenation and thus to disruption of sleep. Apneas are traditionally (but confusingly) categorized as either central, where there is no respiratory effort, or obstructive, where there is respiratory effort. With some central apneas, the airway is open, and the subject is merely not attempting to breathe. Conversely, with other central apneas and all obstructive apneas, the airway is closed. The occlusion is usually at the level of the tongue or soft palate. The airway may also be partially obstructed (i.e. narrowed or partially patent). This also leads to decreased ventilation (hypopnea), decreased blood oxygenation and disturbed sleep.
The common form of treatment of these syndromes is the administration of Continuous Positive Airway Pressure (CPAP). The procedure for administering CPAP treatment has been well documented in both the technical and patent literature. An early description can be found in U.S. Pat. No. 4,944,310 (Sullivan). Briefly stated, CPAP treatment acts as a pneumatic splint of the airway by the provision of a positive pressure, usually in the range 4-20 cm H2O. The air is supplied to the airway by a motor driven blower whose outlet passes via an air delivery hose to a nose (or nose and/or mouth) mask sealingly engaged to a patient""s face. An exhaust port is provided in the delivery tube proximate to the mask.
Various techniques are known for sensing and detecting abnormal breathing patterns indicative of obstructed breathing. U.S. Pat. No. 5,245,995 (Sullivan et al), for example, describes how snoring and abnormal breathing patterns can be detected by inspiration and expiration pressure measurements while sleeping, thereby leading to early indication of preobstructive episodes or other forms of breathing disorder. Particularly, patterns of respiratory parameters are monitored, and CPAP pressure is raised on the detection of pre-defined patterns to provide increased airway pressure to, ideally, subvert the occurrence of the obstructive episodes and the other forms of breathing disorder.
Automatic detection of partial upper airway obstruction and pre-emptive adjustment of nasal CPAP pressure works to prevent frank obstructive apneas in the majority of subjects with obstructive sleep apnea syndrome. However, some subjects with severe disease progress directly from a stable open upper airway to a closed airway apnea with complete airway closure, with little or no intervening period of partial obstruction. Therefore it is useful for an automatically adjusting CPAP system to also respond to a closed airway apnea by an increase in CPAP pressure. However, it is not desirable to increase CPAP pressure in response to open airway apneas, firstly because this leads to an unnecessarily high pressure, and secondly because the high pressure can reflexly cause yet further open airway apneas, leading to a vicious circle of pressure increase.
One method for distinguishing open airway apneas (requiring no increase in pressure) from closed airway apneas (requiring a pressure increase) is disclosed in European Publication No. 0 651 971 A1. During an apnea, the mask pressure is modulated at 4 Hz with an amplitude of the order of 1 cmH2O the induced airflow at 4 Hz is measured, and the conductance of the airway is calculated. A high conductance indicates an open airway. This forced oscillation method requires the ability to modulate the mask pressure at 4 Hz, which increases the cost of the device. Furthermore, the method does not work in the presence of high leak, and can falsely report that the airway is closed if the subject has a high nasal or intrapulmonary resistance.
The present invention is directed to overcoming or at least ameliorating one or more of the foregoing disadvantages in the prior art.
Therefore, the invention discloses a method for the administration of CPAP treatment pressure comprising the steps of:
supplying breathable gas to the patient""s airway at an initial treatment pressure, and repeatedly:
determining a measure of respiratory airflow;
determining the occurrence of an apnea from a reduction in said measure of respiratory airflow below a threshold;
determining the duration of said apnea; and
increasing the treatment pressure by an amount which is an increasing function of the duration of said apnea, and a decreasing function of the treatment pressure immediately before said apnea.
The invention further discloses CPAP treatment apparatus comprising:
a controllable flow generator operable to produce breathable gas at a pressure elevated at a pressure above atmosphere;
a gas delivery tube coupled to the flow generator;
a patient mask coupled to the tube to receive said breathable gas from the flow generator and provide said gas, at a desired treatment pressure, to the patient""s airway;
a controller operable to receive input signals and to control operation of said flow generator and hence the treatment pressure; and
sensor means located at the flow generator, in the delivery tube or at the mask that generates a signal input to said controller from which patient respiratory airflow is determined;
and wherein said controller is operable to determine the occurrence of an apnea from a reduction in said respiratory airflow, determine the duration of said apnea, and cause said flow generator to increase CPAP treatment pressure by an increment that is an increasing function of said apnea duration and a decreasing function of the treatment pressure immediately prior to said apnea.
In a preferred form, said sensor means can comprise a flow sensor, and said controller derives respiratory airflow therefrom.
In one preferred form said initial treatment pressure is 4 cmH2O said measure of respiratory airflow is the two second moving average RMS airflow, and said threshold is 25% of the RMS airflow over the preceding 5 minutes. In this preferred form, no increase in pressure is made for apneas of less than 10 seconds duration, or for apneas where the treatment pressure immediately prior to the apnea is more than 10 cmH2O but otherwise, the lower the treatment pressure immediately prior to the apnea, and the longer the apnea, the greater the increase in treatment pressure, up to a maximum of 8 cmH2O per minute of apnea. In this preferred form, if there is no apnea the treatment pressure is gradually reduced towards the initial minimum pressure with a time constant of 20 minutes.
The method and apparatus can advantageously be used in concert with one or more other methods for determining the occurrence of partial upper airway obstruction, such that either complete or partial upper airway obstruction can lead to an increase in pressure, but once there is no longer either complete or partial obstruction, the pressure will gradually reduce towards the initial minimum pressure.
In one particularly preferred form, partial obstruction is detected as either the presence of snoring, or the presence of characteristic changes in the shape of the inspiratory flow-vs-time curve indicative of inspiratory airflow limitation.
The method and apparatus can also advantageously be used in concert with a forced oscillation method for measuring airway patency, in which the CPAP pressure is modulated with an amplitude of, for example, 1 cmH2O at 4 Hz, the induced airflow at 4 Hz is measured, the conductance of the airway calculated by dividing the amplitude of the induced airflow by the pressure modulation amplitude, and the additional requirement imposed that the treatment pressure is only increased if said conductance is greater than a threshold.
Closed airway apneas are most likely to occur at low CPAP pressures, because high CPAP pressures splint the airway partially or completely open, whereas pressure-induced open airway apneas are most likely to occur at high CPAP pressures, at least partially because high CPAP pressures increase lung volume and thereby stimulate the Hering-Breuer reflex, leading to inhibition of breathing. Therefore, the lower the existing CPAP pressure, the more likely an apnea is to be of the closed airway variety, and the more appropriate it is to increase the treatment pressure, whereas the higher the existing CPAP pressure, the more likely an apnea is to be of the open airway variety, and the more appropriate it is to leave the CPAP pressure unchanged. Generally apneas of less than 10 seconds duration are regarded as non-pathological, and there is no need to increase CPAP pressure, whereas very long apneas require treatment. The present invention will correctly increase the CPAP pressure for most closed airway apneas, and correctly leave the CPAP pressure unchanged for most open airway apneas.
The present invention can be combined with an independent pressure increase in response to indicators of partial upper airway obstruction such as snoring or changes in shape of the inspiratory flow-time curve. In this way it is possible in most subjects to achieve pre-emptive control of the upper airway, with pressure increases in response to partial upper airway obstruction preventing the occurrence of closed airway apneas. In the minority of subjects in whom pre-emptive control is not achieved, this combination will also correctly increase the CPAP pressure in response to those closed airway apneas that occur at low CPAP pressure without prior snoring or changes in the shape of the inspiratory flow-time curve. Furthermore, the combination will avoid falsely increasing the CPAP pressure in response to open airway apneas induced by high pressure.
Some open airway apneas can occur at low pressure. By combining the forced oscillation method with the present invention, with the additional requirement that there be no increase in pressure if the forced oscillation method detects an open airway, false increases in pressure in response to open airway apneas at low pressure will be largely avoided.